1. Field of the Invention
This invention concerns an extrusion or coating die having a removable insert for changing the flow characteristics of the extruded fluid stream.
2. Description of the Related Art
A variety of goods are manufactured by processes that involve an extrusion or coating die. Some dies, for example, are used to form thin films, bars, or other elongated shapes of plastic material. Other dies are widely used to apply a coating of fluid material to a moving web.
One example of an item made by coating a moving web is adhesive tape. Many types of adhesive tape are made by spraying a liquid adhesive onto a plastic substrate that serves as a backing for the tape. Other examples of items made by coating a web include photographic film, coated paper and magnetic audio, video and data storage tapes. Another example is orthopedic splinting/casting tape, such as Scotchcast.TM. brand tape or Scotchcast Plus.TM. brand tape (both from 3M) which include a porous fiberglass backing that is coated with a water curable polyurethane resin.
The application of a liquid or semi-liquid material to a moving web is carried out in some instances by using a coating die. The die receives the material under pressure from a pump or other device, and distributes the fluid material in regions across the width of the web as the web is advanced along its longitudinal axis. The die may include an outlet slot that discharges the fluid in a ribbon onto the web, or alternatively may include a series of discharge nozzles that are arranged to distribute the fluid to various regions across the width of the web.
Conventional coating dies normally have an inlet passageway, an outlet passageway and an inner chamber that extends between the inlet passageway and the outlet passageway. The outlet passageway is relatively wide and often approximates the width of the web in instances where the die has a slot-type outlet for discharging a ribbon of fluid onto the web. The inner chamber is also relatively wide, and serves as a manifold to distribute incoming fluid from the inlet passageway to various regions of the outlet passageway.
Individuals who are skilled in the art of coating and extrusion die design sometimes refer to the "profile" provided by a particular die. The "profile" is a graphical representation of the flow rate of fluid measured at various locations across the width of the die outlet. For example, in some applications' a die with a flat profile is desired. A flat profile indicates that the flow rate of fluid is the same wherever it is measured along the width of the outlet. In other instances, a die having a parabolic profile is desired, which indicates that the flow rate of fluid is greatest at the center of the die outlet and decreases as either side of the die outlet is approached.
Traditionally, many coating and extrusion dies have had a triangular or "coat hanger" shape with a central inlet passageway, a relatively wide outlet passageway and an inner manifold chamber having a triangular shape to distribute the incoming fluid to various regions of the outlet passageway. However, because the inlet passageway is located directly across from the center of the outlet passageway, the flow rate of fluid in central portions of the outlet passageway tends to be greater than the flow rate of fluid near the ends of the outlet passageway. While such a profile may be suitable for some applications, it is unsuitable for other applications such as, for instance, when a die having a flat profile is desired.
A number of approaches have been suggested in the past for altering the profile of coat hanger dies. One approach is to vary the cross-sectional area of the inner chamber (in directions perpendicular to the flow of fluid) as either end of the chamber is approached. For example, by increasing the cross-sectional area of the chamber as either end is approached, the flow rate of fluid near the sides of the outlet passageway can be increased.
Coat hanger dies are typically made by machining two blocks of metal to make separable die halves. The inlet passageway, the outlet passageway and the inner chamber are formed in one or both of the die halves. If a different profile is needed, the shape of the chamber is altered by disassembling the die halves and re-machining one or both of the die halves. The shape of the chamber may also be altered by affixing one or more shims to side walls of the chamber in order to decrease the size of the chamber where desired.
Unfortunately, it is often difficult to design a die that provides a particular profile that is achieved the first time the die is put in service. In practice, a test run of a new die is conducted to observe its profile. Following the run, the two die halves are disassembled and the inner chamber and/or the shims are machined in an attempt to compensate for any deficiency of the profile. The die is then reassembled and the profile again observed.
It can be appreciated that the trial and error technique of adjusting the profile of a coat hanger die is somewhat time consuming and expensive. In addition, the profile may change when other process conditions are changed, such as the viscosity of the fluid or the total flow rate of fluid.
U.S. Pat. No. 5,234,330 describes a coat hanger die having two separable die halves and one or more inserts that are located in an inner chamber of the die when the die halves are assembled. The inserts vary in cross-sectional area along their length for increasing or decreasing adjacent cross-sectional areas of the chamber such that the profile of the die is affected.
There is a continuing need in the art to improve known coating and extrusion dies, so that the cost of constructing the die can be reduced. There is also a need to provide a die that is easy to clean when necessary without undue effort. Preferably, such a die would be adaptable for use with a wide range of liquids and semi-liquid materials and also under various process conditions.